For the last three years, I've been urging people to experiment with their dosing, trying lower and higher weights of coffee. I was (slightly) wrong; I should have been urging people to experiment with grind settings, trying finer and coarser grinds.
Six of one, half dozen of the other? Not really. Suppose you had a great shot yesterday at a certain grind and dose. The next day, the shot chokes or goes too fast. Here's my argument: change the grind and keep your dose the same, and the shot will no longer be great; keeping the grind the same, and change your dose, and the shot will stay great.
I'll illustrate this with an example; and then make an argument on why this is so.
I'm comparing roasts of the same coffee with Ken Fox. Today I was comparing the Agua Tibia, a Fraijanes we got from SM. The roast color levels were close to the same, both whole bean and ground, indicating functionally equivalent roast profiles. When the coffees first arrived, the grind settings were also the same. But today, a typical air versus drum roast difference appeared -- put the beans on a cutting board, and slice them with a chef's knife, the drum roasted beans tend to shatter more than slice, while the airroasted ones slice more than shatter. This means the drum roasted beans created more fines and the same dose and grind flowed slower than those from the air roaster.
As it happens, Ken's drum roast flowed right, and tasted like Kirsch and Kahlua; while mine flowed fast and tasted sour. The Kirsch and Kahlua thing is the reason we bought these beans. I might have screwed up the roast, but first I needed to make a proper shot. When I tightened up the grind, and maintained the same dose, what I got was overly bitter. When I raised the dose to get the right flow, and kept the grind the same, the shot tasted as it should.
In espresso extraction, the flow rate is largely determined by the amount of fines in the puck, while the extraction rate is determined by the average size of the coarse particles. When you tighten the grind, you do two things: create more fines, slowing down the flow, and make the average size of the coarse particles smaller, raising the extraction rate. This means that if the flow rate is wrong, and the extraction rate is right, changing the grind will make the flow rate right at the cost of screwing up the extraction rate. Once the extraction rate is right, the grind setting should be sacrosanct, and you should control the flow rate by changing the dose.